In the monitoring of oil and coal combustion processes, the gases flowing within a stack or duct into the atmosphere are examined optically for content. The temperature within the duct is within the range of 90.degree. C. to 260.degree. C. with gas velocities up to 18 meters/second. The duct gases contain acid and water vapors as well as condensation and particulate matter. These conditions made in situ monitoring of the duct gases impractical. In practice, therefore, the duct gases have been monitored using an optical detector mounted on the outer surface of the duct at a duct opening. The detector is exposed to the duct gases at the opening through a window unit that tends to become fouled by the particulate content of the gases. In order to maintain constant the sensitivity of the monitoring system, the windows must be periodically cleaned; cleaning in practice is required following one to four hours of operation.
One system for reducing fouling of the window unit in a duct monitoring system using optical instrumentation involves applying a purge gas to the window to minimize contact of the particulate matter with the window surface. An exemplary system of this type is shown in U.S. Pat. No. 3,847,487 to Boll. Although generally somewhat satisfactory, prior art purge gas systems typically require introduction of a substantial amount of the gas. Because there is a violent movement of the gas in proximity to the window and duct opening, the normal flow of duct gases is significantly altered. The sensitivity of the instrument is reduced because the particulate matter tends to be deflected deeply into the duct out of the field of view of the optical detector.
Another technique for reducing window fouling is to transform the housing on which the window unit is mounted to a number of smaller cylinders having a large length-to-diameter ratio. This technique, used in the conventional "Everclean" window developed by C. E. R. L., is based upon the principal that dust will not be deposited at the end of a cylinder at right angles to the duct if the ratio of the length-to-diameter is at least 4:1 for coarse particles and 16:1 for finer dust and fumes. A disadvantage of the "Everclean" approach, however, is that the large length-to-diameter ratio of the cylinders causes the resulting housing to be excessively long in some applications. An even more serious deficiency is that the long narrow cylinders tend to pass only light parallel to or only slightly diverging from the axes of the cylinders; widely diverging light is blocked from the optical detectors by the cylinder walls.
It is accordingly one object of the present invention to provide a new and improved system for minimizing fouling of duct viewing windows.
Another object is to provide a system for reducing fouling of viewing windows using less purge gas than previously required.
A further object is to provide a new and improved system for reducing fouling of duct gas viewing windows wherein there is minimum interference with flow of duct gases on the viewing axis.
Another object is to provide a new and improved system for minimizing fouling of duct gas viewing windows wherein there is minimum attenuation of light widely diverging from the viewing axis of the window.
An additional object is to provide a purge gas system for reducing fouling of duct gas viewing windows that is more compact tham similar systems of the prior art.